In a speech, Obama took Republicans to task, in particular 11 GOP senators who supported recent efforts to improve the immigration system. He did not name any, but told his largely supportive audience at American University that those lawmakers had succumbed to the "pressures of partisanship and election-year politics."

Seeking to build new momentum on an issue many advocates hoped would be resolved by this point, Obama laid out his rationale for a comprehensive approach to fixing what he and others, Republicans included, say is a broken immigration system.

He said the problem cannot be solved "only with fences and border patrols" but said the government should be held accountable for its responsibility to secure the border. Obama also said that businesses should face consequences for knowingly employing illegal immigrants. And he said those who enter the country illegally should own up to their actions before they can begin the process of becoming citizens.

"The question now is whether we will have the courage and the political will to pass a bill through Congress, to finally get it done," the president said. "I'm ready to move forward, the majority of Democrats are ready to move forward and I believe the majority of Americans are ready to move forward. But the fact is that without bipartisan support, as we had just a few years ago, we cannot solve this problem."

"Reform that brings accountability to our immigration system cannot pass without Republican votes," he said. "That is the political and mathematical reality."

In response, Arizona Sen. Jon Kyl, one of the 11 Republican senators Obama alluded to in his talk, said he had a good reason for his position this time around.

"My constituents have said do everything you can to secure the border first," Kyl told Fox News Channel. "It's our job to secure the border, whether or not we end up passing so-called comprehensive immigration reform."

White House officials say recent developments influenced Obama's decision to give his first formal speech on the issue as president, most notably Arizona's enactment of a tough anti-immigrant law and the reaction to it across the country. But advocates also have been pressing him to give such a speech as a demonstration of his commitment to seeing the effort through.

Obama didn't dwell on the Arizona law in the speech. He called it an understandable byproduct of public frustration with the government's inability to tighten the system, but also said the law is ill-conceived, divisive and would put undue pressure on local authorities.

The law requires police enforcing another statute to clarify a person's immigration status if there's reason to believe that person is in the U.S. illegally. Immigrant advocates want the Justice Department, which is reviewing the law, to sue Arizona to block it from taking effect this month.

In the speech, Obama extolled America's history as a melting pot of immigrants and lauded their many contributions to the nation.

But an Associated Press-GfK Poll conducted in May found 57 percent saying illegal immigrants are mostly a drain on society and 38 percent said they believe immigrants make a contribution. Eight in 10 said the federal government should do more to keep immigrants from illegally entering the U.S.

Obama has endorsed a proposal by Sens. Chuck Schumer, D-N.Y., and Lindsey Graham, R-S.C., that would require illegal immigrants to admit they broke the law, pay fines and back taxes and perform community service to eventually obtain legal status. But Graham since has balked at acting on immigration this year, and no other Senate Republican has come forward.

Some Republicans, like Kyl, are pushing a "border security first" approach focused on enforcement.

"It won't work," Obama said. He said there now are more "boots on the ground" on the U.S.-Mexico border than ever before and that "our borders are just too vast for us to be able to solve the problem only with fences and border patrols."

Obama recently ordered 1,200 National Guard troops to the border to boost security and asked Congress for an additional $600 million to support personnel and improve technology there. More than 500 of those Guard troops are headed for Arizona.

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